1. Field of the Invention
The present invention relates to a gain control circuit that controls an audio output reproduced by an FM receiver according to the magnitude of a noise component, an FM receiver including such a gain control circuit, and a computer program product that implements a function of such a gain control circuit.
2. Description of Related Art
With an FM receiver, if a radio wave input into the antenna has a predetermined electric field strength or higher, an audio signal demodulated by the FM detector will be less influenced by pulse noise, fading and the like, and thus a high quality audio output with a high S/N ratio can be obtained.
However, the output level of the demodulated signal is constant irrespective of the received electric field strength. Accordingly, if the received radio wave is weak (or in other words, the electric field strength is low), the demodulated audio will be weak, and significant noise will occur in the audio output.
Conventionally, two methods have been used primarily as methods for reducing discomfort due to such unpleasant noise (see JP 8-18468A).
One is a method (hereinafter referred to as the “first method”) in which an audio output from a speaker by a squelch circuit is cut off (see FIG. 4 and the corresponding description of the above-mentioned Patent Document). Specifically, only a noise component of a detected demodulated signal is amplified and rectified by a squelch amplifier, and when the amount of the noise component is large, a squelch switch is controlled to turn off an output of a preamplifier and cut off the audio output from the speaker so as to be silent.
The other is a method (hereinafter referred to as the “second method”) in which a gain control circuit including an electric field strength detection means and a preamplifier is disposed on the output side of an FM detector (see FIG. 1 and the corresponding description of the above-mentioned Patent Document). Specifically, a control signal according to the electric field strength of a radio wave received by the electric field strength detection means is generated, and the output of the preamplifier is reduced according to the intensity of the control signal. If the electric field strength of the received radio wave is small, the output of the preamplifier will be small, as a result of which the noise level of reproduced audio decreases according to the electric field strength.
In the case where the above-described first method is used in an FM receiver for wireless communication, the squelch is slightly opened when radio wave propagation is poor or when the radio wave is weak due to the distance to communication partner being too far. In such a case, there is a problem in that the user has to listen to an intended signal amid significant noise until the squelch is operated.
In the case where the above-described second method is used, not only when the electric field strength of an intended signal included in the received signal is high, but also when the electric field strength of the intended signal is low and the noise component is large, it is determined that the electric field strength is high due to an increase in the power level of the noise component, and the reproduced audio including a large amount of noise component is output from the speaker. Conversely, when the electric field strength of the intended signal included in the received signal is low, there is a problem in that the reproduced audio output is reduced too much, irrespective of the magnitude of noise component.